pgsql: Further cleanup of gistsplit.c.

Further cleanup of gistsplit.c.

After further reflection I was unconvinced that the existing coding is
guaranteed to return valid union datums in every code path for multi-column
indexes. Fix that by forcing a gistunionsubkey() call at the end of the
recursion. Having done that, we can remove some clearly-redundant calls
elsewhere. This should be a little faster for multi-column indexes (since
the previous coding would uselessly do such a call for each column while
unwinding the recursion), as well as much harder to break.

Also, simplify the handling of cases where one side or the other of a
primary split contains only don't-care tuples. The previous coding used a
very ugly hack in removeDontCares() that essentially forced one random
tuple to be treated as non-don't-care, providing a random initial choice of
seed datum for the secondary split. It seems unlikely that that method
will give better-than-random splits. Instead, treat such a split as
degenerate and just let the next column determine the split, the same way
that we handle fully degenerate cases where the two sides produce identical
union datums.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/639f5adb97801539b2bd7c35b9e20b14e034d763

Modified Files
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src/backend/access/gist/gistsplit.c | 157 +++++++++++++++++++++--------------
1 files changed, 93 insertions(+), 64 deletions(-)